Conjugated polyelectrolyte capsules: light activated antimicrobials

ABSTRACT

Hollow conjugated polyelectrolyte (HCPE) microcapsules contain at least one conjugated polyelectrolyte and at least one other polyelectrolyte of complementary charge and the microcapsule has a hollow core. The conjugated polyelectrolyte is a polymer with a multiplicity of charged repeating units where a portion of the charged repeating units form a pi-conjugated sequence. The complementary polyelectrolyte is a polymer with a complementary charged repeating unit to the charged repeating units of the conjugated polyelectrolyte. The HCPE microcapsules can be formed by successively coating a sacrificial core with alternating layers of complementary polyelectrolytes, at least one of which is a conjugated polyelectrolyte. The sacrificial core can be removed to form the hollow center of a HCPE microcapsule. The HCPE microcapsules can be contacted with a medium containing microbes where the HCPE microcapsules associate with the microbes and efficiently kill the microbes when irradiated with light or other electromagnetic radiation.

CROSS-REFERENCE TO A RELATED APPLICATIONS

This application is the U.S. national stage application of InternationalPatent Application No. PCT/US2009/063715. filed Nov. 9, 2009, whichclaims the benefit of U.S. Provisional Application Ser. No. 61/112,867,filed Nov. 10, 2008, the disclosures of which are hereby incorporated byreference in their entireties, including all figures, tables ordrawings.

The subject invention was made with government support under the ArmyResearch Office, Contract No. W911NF-07-1-0079. The government hascertain rights to this invention.

BACKGROUND OF THE INVENTION

Surfaces with antimicrobial properties are of interest for medicalfacilities, medical devices, and even homes and household items toreduce the transmission of diseases and other malaise from one organismto another organism through common contact with the surface. Microbesgenerally have a limited lifetime on a surface; however that lifetime isgenerally for at least a few hours and is often in the order of days.Even when the microbes survive for hours significant contact can occurover that period. The most prevalent technologies used for antimicrobialsurfaces include: silver ion technology; organic biocides in coatings orincorporated into objects; and cationic biocides covalently bonded to areactive silicone compounds.

Silver ion technology in its simplest form is a coating system formed bybinding silver ions to a fine ceramic powder, dispersed in a carrier.The silver ions, which can interrupt critical functions ofmicroorganisms, are then exchanged with other ions when the surfacecomes into contact with aqueous fluids. This technology has been used totreat steel ducting and components in HVAC systems, building materials,laminates, floors, walls, paints, carpets, cubicle curtains, lockers,safety cabinets, bedpans, sack holders, soap dispensers, keypads,medical devices, wound dressings and implants.

Organic biocides have been incorporated into plastic and fabrics duringmanufacture. The concentration of biocide is optimized to provideadequate protection. Many different organic biocides are now in use,some specially selected for specific applications. Some of the earliestapplications involved combating the growth of odor causing bacteria onmedical devices. Applications for the building industry include walllaminates, for hospital environments include bedding, bed frames, cartsfor transport of medication, linens, hand rails, pulls, trims, doorhandles, slings, hoists, window blinds, dental trays, incise drapes,wound dressings, keyboards and other equipment.

Biocides covalently bonded to a reactive silicone compound are fixedonto a surface as an extremely thin film with little effect to thephysical properties of the surface. Fixation of the biocide is importantwhere toxicity, taint and other organoleptic aspects are of concern.This method is a bacteriostatic surface treatment that allows andrequires normal cleaning procedures to maintain the biocidal activity ofthe surface. The technology has been used for treatment of bedsheets,hospital garments, curtains, floor and wall materials, air filtrationsystems, medical devices, bandages, surgical instruments and implants.The technique has been used to prevent biofilm growth on catheters,stents, contact lenses and endotracheal tubes.

These technologies can suffer from toxicity to organisms other than themicrobes or promote formation of resistant strains of the microbes.Recently, light activated antimicrobial agents such as the dyeindocyanine green have been found to be highly potent only when lightactivated, harmless to the organism and not believed to promoteresistance due to the light only toxicity to the microbes. Polymericmaterials generally display low toxicity to organisms and can be readilyfixed to surfaces. Light activated antimicrobial polymeric materialshave been reported, generally consisting of small moleculeantimicrobials included in a polymeric carrier, but are not covalentlybonded to the polymer. Antimicrobial polymers have the potential toexist in many different structural forms where enhancements, such as thesurface area to volume, can significantly increase their activity towardmicrobes relative to surface fixed antimicrobial agent. Hence, a polymerthat is a high molecular weight light activated antimicrobial agent isan attractive solution to many limitations of present antimicrobialsystem.

BRIEF SUMMARY OF THE INVENTION

An embodiment of the invention is a hollow conjugated polyelectrolyte(HOPE) microcapsule where at least one conjugated polyelectrolyte is inassociation with at least one complementary polyelectrolyte in the shapeof a microcapsule with a hollow core. The conjugated polyelectrolyte isa polymer with a multiplicity of charged repeating units with at least aportion of the charged repeating units forming a pi-conjugated sequence.The complementary polyelectrolyte is one where a plurality of repeatingunits has a charge that is opposite the charge on the repeating units ofthe conjugated polyelectrolyte, such that charges on thepolyelectrolytes associate through ion-pairing of some of the repeatingunits of the two polyelectrolytes. The complementary polyelectrolytescan also be a conjugated polyelectrolyte. The HCPE microcapsules can be0.5 to 100 μm in cross-section, for example about 4 to about 6 μm incross-section, and can be monodispersed in size distribution. In oneembodiment of the invention, the HCPE microcapsule can be formed fromthe complementary conjugated polyelectrolytes PPE-SO₃ ⁻ and PPE-4⁺. Aplurality of polyelectrolytes can exist as a plurality of layers whereadjacent layers have complementary charges on the polymer.

Another embodiment of the invention is a method of preparing (HCPE)microcapsules where a sacrificial core is coated with a polyelectrolytelayer and additionally coated on the exterior polyelectrolyte layer withan additional polyelectrolyte layer. The additional polyelectrolyte hasrepeating units of a complementary charge to the polyelectrolyte uponwhich it is deposited. Optionally, successive additional coatings ofadditional complementary polyelectrolytes can be deposited where thedeposited polyelectrolyte has repeating units of a complementary chargeto the polyelectrolyte upon which it is deposited. At least one of thepolyelectrolytes used to form the HCPE microcapsule is a conjugatedpolyelectrolyte. After deposition of the layers, the sacrificial corescan be removed to leave the (HCPE) microcapsules.

In one embodiment of the invention, the sacrificial core is a saltparticle with low water solubility. After formation of the conjugatedpolyelectrolyte (CPE) microcapsule, the core can be removed bydissolving a species formed from the salt particle by contacting thesalt particle with an acid, base, metal complexing agent or anycombination thereof to form the HCPE microparticle. For example, amonodispersed MnCO₃ particle can be prepared and a PPE-SO₃ ⁻ conjugatedpolyelectrolyte can be coated from aqueous solution on the MnCO₃ core, acomplementary PPE-4⁺ conjugated polyelectrolyte can be coated fromaqueous solution on the PPE-SO₃ ⁻ conjugated polyelectrolyte and EDTAcan be added to decompose the MnCO₃ sacrificial core and dissolve theresulting complexed MnEDTA salt.

In another embodiment of the invention, (HCPE) microcapsules can be usedin a method for killing microbes where a plurality of hollow conjugatedpolyelectrolyte (HCPE) microcapsules is contacted with a mediumsuspected of containing microbes and irradiating the (HCPE)microcapsules with visible light, ultraviolet light or otherelectromagnetic radiation to kill the microbes contacted by one or moreof the polyelectrolyte (HCPE) microcapsules. The medium can be anaqueous medium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows scanning electron and fluorescence microscope images atdifferent stages of preparation of HCPE microcapsules where: (a) shows aSEM image of a MnCO₃ core before coating; (b) shows a fluorescencemicroscope image of CPE microparticles before removal of the sacrificialcores; and (c) shows a fluorescence microscope image of a flattened HCPEmicrocapsule with a creased surface; (d) shows a SEM image of aflattened HCPE microcapsule with a creased surface.

FIG. 2 shows the chemical structure of an anionic conjugatedpolyelectrolyte (PPE-SO₃ ⁻) according to an embodiment of the inventionand a cationic conjugated polyelectrolyte (PPE-4⁺) according to anembodiment of the invention.

FIG. 3 shows confocal microscope images of HCPE microcapsules with (a)live trapped bacteria before irradiation and (b) dead trapped bacteriaafter 15 minutes of irradiation.

FIG. 4 shows a central slice of large clusters with entrapped deadbacteria after 1 hour exposure to white light.

FIG. 5 shows HCPE microcapsules with entrapped bacteria where themicrocapsules display (a) fibrillar structures, filaments, and (b) and(c) sheet-like structures.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention are directed to hollow conjugatedpolyelectrolyte (HCPE) microcapsules that have antimicrobial propertieswhen irradiated with electromagnetic radiation. In an embodiment of theinvention, the HCPE microcapsules display effective antimicrobial actionagainst bacteria, viruses, fungi or other microbes that associate withthe HCPE microcapsules, which acts as an efficient trap where thetrapped microbes are efficiently killed upon irradiation. The microbescan be entrapped within or attached to the external surface of the HCPEmicrocapsules. Not to be bound by a theoretical mechanism, thelight-activated antimicrobial activity is consistent with the generationof singlet oxygen and possibly successor reactive oxygen intermediatesat the photoexcited HCPE microcapsule upon irradiation.

In one embodiment of the invention, the HCPE microcapsules comprise aplurality of layers of polyelectrolytes of complementary charge where atleast one polyelectrolyte is a conjugated polyelectrolyte. The layersare formed about a sacrificial core. In one embodiment of the inventionthe sacrificial cores are monodispersed in size to allow the formationof monodispersed HCPE microcapsules upon removal of the sacrificialcores. The conjugated polyelectrolytes can be adsorbed on thesacrificial cores in a layer-by-layer fashion were the first layer canbe either a cationic polyelectrolyte or an anionic polyelectrolyte.Although, two layers are sufficient to form the HCPE microcapsules, inother embodiments of the invention additional layers can be formed toyield multilayer HCPE microcapsules. In other embodiments of theinvention, one or more conjugated polyelectrolytes can be bound to anon-conjugated polyelectrolyte, where the conjugated polyelectrolyte iscapable of contacting an external bacteria or other microbial agent.

The sacrificial cores can be of any material that effectively associateswith a polyelectrolyte and can be removed after construction to form theHCPE microcapsules. The core can be any particulate material orsuspended liquid droplet that can promote the formation of a layer of ananionic or cationic polyelectrolyte on its surface, yet can be removedafter formation of the desired number of polyelectrolyte layers on thesurface. The core can be removed in any fashion that does not damage theHCPE microcapsules, for example the removal can be any method ofdissolving, vaporizing, and/or degradation of the sacrificial core. Inone embodiment of the invention the sacrificial core is relativelymonodispersed in size. The monodispersed cores have non aggregated coresthat have a standard deviation in size that is less than about 25% ofthe mean size, for example, less than 10% of the mean size.

In one embodiment of the invention, the sacrificial core is a saltparticle that is of low solubility in water. Salts that can be used asthe sacrificial cores include, but are not restricted to, carbonates,oxides, chalcogenides, and other salts. Useful salts include those thatcan be decomposed using an acid or base, or dissolved using a metalcomplexing agent, such as ethylenediaminetetraacetic acid (EDTA). Thesalt particle can be prepared by any method where the size can berestricted to a particular desired size range, for example less than 100μm, for example 50, 45, 40, 35, 30, 25, 20, 15, 10, 9, 8, 7, 6, 5, 4, 3,2, 1, 0.5 or 0.1 μm. Where, in the lower limits, what is referred to asa microcapsule for purposes of the invention can be a nanocapsule wherethe size is less than 1 μm. The particles can be selected by size usingsieves for isolation of a particular size distribution out of a sourcethat is polydispersed in size or particles of a selected size and/ordistribution can be achieved by direct formation of particles by anymethod where nucleation and propagation of particle formation can becontrolled. For example, monodispersed metal carbonate particles, forexample MnCO₃ particles, can be formed as taught in Zhu et al., Chem.Mater. 2005, 17, (9), 2323-8, and is incorporated herein by reference.For example, the standard deviation in size can be less than 10% of amean size that can be, for example, less than 10 μm in size. To removeMnCO₃ particles and leave HCPE microcapsules, decomposition anddissolution of the MnCO₃ with an EDTA solution results in the loss ofCO₂ with the liberation of MnEDTA to the aqueous solution.

According to embodiments of the invention, the conjugatedpolyelectrolytes can be poly(phenylene ethynylene), poly(phenylenevinylene), poly(acetylene), poly(phenylene), poly(pyrrole), poly(furan),poly(thiophene) or any other conjugated polymer or copolymer where asufficient number of side groups include non-hydrolyzable ionic sidegroups where each ionic side group can contain one or more atoms havinga unit charge centered. The side groups can contain anionic groups, suchas sulfonate, phosphonate, carboxylate, oxide, sulfide, sulfinate,phosphonate, thiosulfate, or any other anionic group that can beattached via a substituted or unsubstituted linking group, such as analkylene, phenylene, or alkylene or phenylene group including one ormore heteroatoms that links the ionic group to the polymer backbone in aregular or random manner. The side groups can contain cationic groups,such as ammonium, pyridinium, phosphonium, or metal ions captured by acrown ether, cryptand, or carcerand units or any other cationic groupthat can be attached via a substituted or unsubstituted linking group,such as an alkylene, phenylene, or an alkylene or phenylene groupincluding one or more heteroatoms that links the ionic group to thepolymer backbone in a regular or random manner.

In embodiments of the invention an anionic conjugated polyelectrolytecan be deposited as a layer on the sacrificial core, followed bydeposition of a complementary cationic conjugated polyelectrolyte. Therepeating units of the two complementary conjugated polyelectrolyte canbe identical except for the ionic side group, for example with the sameconjugated repeating unit structure, the position of the ionic sidegroups can be equally spaced along the conjugated backbone with anequivalent charge on every side group. In other embodiments of theinvention, the side groups can have unequal charge, for example acationic side group can have a charge of +2 and the paired anionic sidegroup of the complementary conjugated polyelectrolyte can have a chargeof −1. The sacrificial core can stabilize the microcapsule shape suchthat the HCPE microcapsule upon removal of the core can slowly undergo ametamorphosis in shape or the HCPE microcapsule can retain the shaperesulting after deposition of the polyelectrolytes on the core.

In some embodiments of the invention, one of the complementarypolyelectrolytes need not be conjugated. For example, a non-conjugatedanionic polyelectrolyte, such as polystyrene sulfonate, can be pairedwith a cationic conjugated polyelectrolyte, or a non-conjugated cationicpolyelectrolyte, such as poly(N-alkyl-4-vinylpyridinium halide), can bepaired with an anionic conjugated polyelectrolyte.

In embodiments of the invention, contact is made between suspensions ofthe HCPE microcapsules with microbes that result in an association. TheHCPE microcapsules can be isolated, or aggregated into clusters in thesecontacted suspensions. For example, a typical mixture can displaydimeric and other larger clusters of HCPE microcapsules. For exampleclusters of 2, 4 and up to 12 HCPE microcapsules can form. ClusteredHCPE microcapsules can form stronger associations with the microbes. TheHCPE microcapsules can form features that increase the surface area ofthe paired conjugated polymers. For example, a HCPE microcapsule canradiate fibrils in the presence of microbes to increase the surface areaof the HCPE microcapsule. Trapping of microbes can be enhanced by theformation of fibrils or sheet-like structures.

Association of the microbes with the HCPE microcapsules does not resultin high killing of the microbes in the dark. However, upon irradiationwith light, the microbes are very effectively killed, where the strengthof the association and the rate and extent of killing is enhanced byclustering of the HCPE microcapsules and microbes. Irradiation can befor short periods of time to extended periods of time. For example, theirradiation can be for less than 1 minute to more than 4 hours.Irradiation can be carried out concurrently or subsequently to themixing of the HCPE microcapsules and the microbes. Irradiation can becarried out with a broad spectrum of light or with a portion of thespectrum which promotes the killing. Light in the visible or ultravioletrange of the spectrum may be employed. As different HCPE microcapsulescan be generated from various CPEs the necessary range must beestablished for a specific HCPE microcapsule, which can be readilydetermined by normal experimentation by one skilled in the art.

Materials and Methods

Preparation of MnCO₃ Microparticles

Monodispersed MnCO₃ microparticles were prepared where nanoparticlesfrom dilutes salt solutions of MnSO₄ and NH₄HCO₃ were used as“nanoseeds” to initiate crystal growth without significant nucleationduring the crystal growth process such that relatively monodispersedMnCO₃ microparticles were formed. The nanoseeds were suspended in aMnSO₄ solution which was vigorously mixed with a NH₄HCO₃ solution togive either smooth cubical MnCO₃ particles or spherical particlesdepending upon the rate of mixing using a stirrer. The MnCO₃microparticles were 4 to 6 μm in cross section. Typical mean particlesize was 4.5 to 5.0 μm with a standard deviation of 0.3 to 0.5 μm, asindicated by scanning electron microscopy (SEM) and dynamic lightscattering (DLS) analysis. Spherical shaped particles can be seen in theSEM image shown in FIG. 1( a) where the particles display a roughsurface with an aggregation of small rectangular crystals forming thespherical particles. DLS data show monodispersed particles in the 4 to 6μm range where 92 to 98% of the particles are isolated particles and 2to 8% of the particles forming aggregates in the 8 to 12 μm range. Anexemplary DLS trace shows a mean particle size of 4.568 μm with astandard deviation of 0.387 μm representing 98% of the sample with 2%aggregate.

Absorption of CPEs on MnCO₃ Microparticles

CPEs were formed by a Layer-by-Layer deposition technique. Suspensionsof MnCO₃ particles in 0.5 M NaCl were prepared with vigorous stirringand sonication of the suspensions. A CPE, PPE-SO₃ ⁻, shown in FIG. 2,was introduced from solution to the suspension and the suspensionstirred for 15 minutes. After centrifugation and washing of theparticles, with 0.2 M NaCl the particles were resuspended in 0.5 M NaClfollowed by addition of the complementary CPE, PPE-4⁺, shown in FIG. 2,solution to the suspension to form a second CPE layer. The formation of4 bilayers on the particles was followed by fluorescence and confocalmicroscopy analysis. Fluorescence microscopy showed emissive CPE layersand uniformly coated microparticles, as can be seen in FIG. 1( b).Confocal microscopy images revealed, in addition to the fluorescentcharacteristics of the particles, uncoated MnCO₃ surfaces refractedlight form particles.

HCPE Formation

Coated particles were treated with a 0.2 M EDTA solution in 1.5 to 3 mLcentrifuge vials. The formation of bubbles occurred upon introduction ofthe EDTA solution. The particles were flushed with solution using apipette and centrifuged after 30 minutes and treated a second time with0.2 M EDTA. Upon complete removal of the MnCO₃, HCPE capsules wereisolated by three cycles of washing with purified water andcentrifugation. Analysis of the HCPE microcapsules by fluorescencemicroscopy with a 100× oil immersion objective resulted in images wheresmall creased features appeared on the HCPE walls, as shown in FIG. 1(c). The HCPE microcapsules assumed a flattened appearance, as shown inFIG. 1( d). The HCPE microcapsules retained the cross section of 4-6 μm.

Preparation of Bacteria and Association with HCPE Microparticles

Pseudomonas aeruginosa and Cobetia marina were raised in a chemostat asdescribed in Chemburu et al., Langmuir 2008, 24, (19), 11053-62.Confocal fluorescence microscopy was used to monitor the association ofthe bacteria with the HCPE microparticles. Bacterial viability wasdetermined by staining with a 1:1 mixture of SYTO 60 and SYTOX greenstains from Molecular Probes. The DNA stains produced red (˜650 nm) andgreen (˜530 nm) emissions for live and dead bacteria respectively. Themixing of a suspension of Pseudomonas aeruginosa with a suspension ofHCPE microcapsules results in the effective capture of the bacteria.FIG. 3( a) shows that bacteria associate on the surface and within theinterior of the HCPE microcapsules. A ratio of more than 7 to 1 wasobserved for live to dead bacteria associated with the HCPEmicrocapsules.

Light Induced Anti-Microbial Action

Irradiation of the HCPE microcapsules associated with the bacteriaresulted in rapid killing of the bacteria. FIG. 3( b) shows a fused pairof HCPE microcapsules associated with Pseudomonas aeruginosa that havebeen irradiated with visible light. The green fluorescence indicatedthat most of the bacteria are dead. When Pseudomonas aeruginosa isassociated with paired HCPE microcapsules visible light irradiationresults in the rapid killing of most bacteria. When HCPE microcapsulesare agglomerated into larger clusters the killing efficiency of the HCPEmicrocapsules is enhanced. FIG. 4 shows large clusters of 4 to 12 HCPEmicrocapsules, particularly abundant are clusters of 4 HCPEmicrocapsules in a tetrahedral orientation to each other. The killing ofall Pseudomonas aeruginosa is apparent in FIG. 3. In all cases oxygenwas present. Otherwise equivalent filled CPE microcapsules show inferiorkilling of microbes upon irradiation.

Structural Evolution of HCPE Microcapsules with Bacteria

Over a period of several hours the HCPE microcapsules undergo astructural evolution that promotes strong association with bacteria.FIG. 5 illustrates that with time fibrillar structures extend from themicrocapsules, as clearly shown in FIG. 5( a). These fibrillarstructures, and sheet-like structures of FIG. 5( b)-(c), are highlyeffective at capturing bacteria. These structures appear to have ahydrogel consistency.

All patents, patent applications, provisional applications, andpublications referred to or cited herein, supra or infra, areincorporated by reference in their entirety, including all figures andtables, to the extent they are not inconsistent with the explicitteachings of this specification.

It should be understood that the examples and embodiments describedherein are for illustrative purposes only and that various modificationsor changes in light thereof will be suggested to persons skilled in theart and are to be included within the spirit and purview of thisapplication.

We claim:
 1. A method of preparing hollow conjugated polyelectrolyte(HCPE) microcapsules comprising: providing a plurality of sacrificialcores wherein said sacrificial cores comprise a salt particle with lowwater solubility; coating said sacrificial cores with a polyelectrolytelayer wherein said polyelectrolyte comprising a multiplicity of chargedrepeating units with a counter ion; additionally coating saidpolyelectrolyte layer with an additional polyelectrolyte layer, whereinsaid additional polyelectrolyte comprising a multiplicity of chargedrepeating units with a counter ion of opposite charge to said counterion of the layer upon which said additional polyelectrolyte layer isadditionally coated; optionally repeating said step of additionallycoating until a desired number of additional polyelectrolyte layers areformed, wherein at least one of said polyelectrolyte or additionalpolyelectrolyte layers comprises a conjugated polyelectrolyte layerwherein a portion of said multiplicity of charged repeating units arepi-conjugated; and removing said sacrificial cores, wherein said step ofremoving comprises dissolving a species formed from said salt particleby contacting said salt particle with an acid, base, metal complexingagent or any combination thereof, wherein a hollow core is formed insaid HCPE microcapsule.
 2. The method of claim 1, wherein said HCPEmicrocapsule comprises said polyelectrolyte layer and one of saidadditional polyelectrolyte layers.
 3. The method of claim 1, whereinsaid salt particle is monodispersed in size.
 4. The method of claim 1,wherein said salt comprises MnCO₃.
 5. The method of claim 1, whereinsaid polyelectrolyte comprises a conjugated polyelectrolyte.
 6. Themethod of claim 5, wherein said conjugated polyelectrolyte comprisesPPE-SO₃ ⁻ or PPE-4⁺.
 7. The method of claim 1, wherein said additionalpolyelectrolyte comprises a conjugated polyelectrolyte.
 8. The method ofclaim 7, wherein said conjugated polyelectrolyte comprises PPE-4⁺ orPPE-SO₃ ⁻.
 9. A method for killing microbes comprising the steps of:contacting HCPE microcapsules prepared according to the method of claim1 with a medium containing microbes; and irradiating said HCPEmicrocapsules in contact with the medium containing microbes withvisible light or ultraviolet radiation, wherein the microbes are killed.10. The method of claim 9, wherein said medium comprising an aqueousmedium.